Process for eliminating surface melt fracture during extrusion of thermoplastic polymers

ABSTRACT

A process and apparatus as provided for substantially eliminating surface melt fracture during extrusion of a thermoplastic polymer such as a molten narrow molecular weight distribution, linear, ethylene copolymer, by using a die having a die land region defining opposing surfaces at least one of which is fabricated from an alloy containing 5 to 95 parts by weight zinc and 95 to 5 parts by weight of copper.

This application is a continuation-in-part of application Ser. No.508,667 filed on June 28, 1983 now abandoned and assigned to a commonassignee.

FIELD OF THE INVENTION

This invention relates to a process and apparatus for substantiallyeliminating melt fracture, particularly surface melt fracture, duringextrusion of thermoplastic polymers susceptible to melt fracture, underconditions of flow rate and melt temperature which would otherwiseproduce such melt fracture.

In a more specific aspect, the invention relates to a process forsubstantially eliminating surface melt fracture during extrusion of amolten narrow molecular weight distribution, linear, ethylene copolymer,under conditions of flow rate and melt temperature which would otherwiseproduce such melt fracture.

BACKGROUND OF THE INVENTION

Most commercial low density polyethylenes are polymerized in heavywalled autoclaves or tubular reactors at pressures as high as 50,000 psiand temperatures up to 300° C. The molecular structure of high pressurelow density polyethylene is highly complex. The permutations in thearrangement of its simple building blocks are essentially infinite. Highpressure resins are characterized by an intricate long chain branchedmolecular architecture. These long chain branches have a dramatic effecton the melt rheology of the resins. High pressure low densitypolyethylene resins also possess a spectrum of short chain branchesgenerally 1 to 6 carbon atoms in length which control resincrystallinity (density). The frequency distribution of these short chainbranches is such that, on the average, most chains possess the sameaverage number of branches. The short chain branching distributioncharacterizing high pressure low density polyethylene can be considerednarrow.

The term "linear" is defined as identifying a polymer chain which ispredominantly free of long chain branchng. By "predominantly free oflong chain branching" is meant less than 0.5 branches/per 1000 carbonatoms in the polyethylene molecule. "Long chain branching" characterizesbranching within polymeric structures which exceeds short branch lengthsof pendant groups derived from individual alpha-olefin comonomers. Along chain branch of polyethylene should have at least a sufficientnumber of carbon atoms to provide significant modifications inrheological behavior, such as caused by chain entanglement. The minimumnumber of carbon atoms is usually greater than about 100. Short chainbranching introduced through comonomer polymerization provides branchlengths of usually less than about 10 carbon atoms per branch.Noncrosslinked LLDPE possesses little, if any, long chain branching suchthat the only branching to speak of is short chain branching, with suchbranch length controlled by the pendant chain length of the comonomericalpha-olefins provided.

The term "narrow molecular weight distribution" as used herein refers tothe ratio of weight average molecular weight to number average molecularweight. This ratio can be between 1 and about 10, preferably betweenabout 2 to about 6.5, and most preferably between about 3 to about 5.The lower limit of this ratio is defined by the theoretical limit sincenumber average molecular weight cannot exceed weight average molecularweight by definition.

Low density polyethylene can exhibit a multitude of properties. It isflexible and has a good balance of mechanical properties such as tensilestrength, impact resistance, burst strength, and tear strength. Inaddition, it retains its strength down to relatively low temperatures.Certain resins do not embrittle at temperatures as low as -70° C. Lowdensity polyethylene has good chemical resistance, and it is relativelyinert to acids, alkalis, and inorganic solutions. It is, however,sensitive to hydrocarbons, halogenated hydrocarbons, and to oils andgreases. Low density polyethylene has excellent dielectric strength.

More than 50% of all low density polyethylene is processed into film.This film is primarily utilized in packaging applications such as formeat, produce, frozen food, ice bags, boilable pouches, textile andpaper products, rack merchandise, industrial liners, shipping sacks,pallet stretch and shrink wrap. Large quantities of wide heavy gaugefilm are used in construction and agriculture.

Most low density polyethylene film is produced by the tubular blown filmextrusion process. Film products made by this process range in size,from tubes which are about two inches or less in diameter, and which areused as sleeves or pouches, to huge bubbles that provide a lay flat ofup to about twenty feet in width, and which, when slit along an edge andopened up, will measure up to about forty feet in width.

Polyethylene can also be produced at low to medium pressures byhomopolymerizing ethylene or copolymerizing ethylene with variousalpha-olefins using heterogeneous catalysts based on transition metalcompounds of variable valence. These resins generally possess little, ifany, long chain branching and the only branching to speak of is shortchain branching. Branch length is controlled by comonomer type. Branchfrequency is controlled by the concentration of comonomer(s) used duringcopolymerization. Branch frequency distribution is influenced by thenature of the transition metal catalyst used during the copolymerizationprocess. The short chain branching distribution characterizingtransition metal catalyzed low density polyethylene can be very broad.

Linear low density polyethylene can also be produced by high pressuretechniques as is known in the prior art.

U.S. Pat. No. 4,302,566 in the names of F. J. Karol et al and entitledPreparation of Ethylene Copolymers in Fluid Bed Reactor, discloses thatethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratioof greater than or equal to 22 to less than or equal to 32 and arelatively low residual catalyst content can be produced in granularform, at relatively high productivities if the monomer(s) arecopolymerized in a gas phase process with a specific high activity Mg-Ticontaining complex catalyst which is blended with an inert carriermaterial.

U.S. Pat. No. 4,302,565 in the names of G. L. Goeke et al and entitledImpregnated Polymerization Catalyst, Process for Preparing, and Use forEthylene Copolymerization discloses that ethylene copolymers, having adensity of 0.91 to 0.96, a melt flow ratio of greater than or equal to22 to less than or equal to 32 and a relatively low residual catalystcontent can be produced in granular form, at relatively highproductivities, if the monomer(s) are copolymerized in a gas phaseprocess with a specific high-activity Mg-Ti-containing complex catalystwhich is impregnated in a porous inert carrier material.

The polymers as produced, for example, by the processes of saidapplications using the Mg-Ti containing complex catalyst possess anarrow molecular weight distribution, Mw/Mn, of about greater than orequal to 2.7 or less than or equal to 4.1.

Low Density Polyethylene: Rheology

The rheology of polymeric materials depends to a large extent onmolecular weight and molecular weight distribution.

In film extrusion, two aspects of rheological behavior are important:shear and extension. Within a film extruder and extrusion die, apolymeric melt undergoes severe shearing deformation. As the extrusionscrew pumps the melt to, and through, the film die, the melt experiencesa wide range of shear rates. Most film extrusion processes are thoughtto expose the melt to shear at rates in the 100-5000 sec⁻¹ range.Polymeric melts are known to exhibit what is commonly termed shearthinning behavior, i.e., non-Newtonian flow behavior. A shear rate isincreased, viscosity (the ratio of shear stress, τ, the shear rate, λ)decreases. The degree of viscosity decrease depends upon the molecularweight, its distribution and molecular configuration, i.e. long chainbranching of the polymeric material. Short chain branching has littleeffect on shear viscosity. In general, high pressure low densitypolyethylenes have a broad molecular weight distribution and showenhanced shear thinning behavior in the shear rate range common to filmextrusion. The narrow molecular weight distribution resins used in thepresent invention exhibit reduced shear thinning behavior at extrusiongrade shear rates. The consequences of these differences are that thenarrow distribution resins used in the present invention require higherpower and develop higher pressures during extrusion than the highpressure low density polyethylene resins of broad molecular weightdistribution and of equivalent average molecular weight.

The rheology of polymeric materials is customarily studied in sheardeformation. In simple shear, the velocity gradient of the deformingresin is perpendicular to the flow direction. The mode of deformation isexperimentally convenient but does not convey the essential informationfor understanding material response in film fabrication processes. Asone can define a shear viscosity in terms of shear stress and shearrate, i.e.:

    η shear=τ.sub.12 /λ

where

η shear=shear viscosity (poise)

τ₁₂ =shear stress (dynes/cm²)

λ=shear rate (sec⁻¹)

an extensional viscosity can be defined in terms of normal stress andstrain rate, i.e.,:

    η ext=π/ε

η ext=extensional viscosity (poise)

π=normal stress (dynes/cm²)

ε=strain rate (sec⁻¹)

During extrusion of a high molecular weight ethylene polymer having anarrow molecular weight distribution through dies, as with other suchpolymeric materials, "melt fracture" occurs when the extrusion rateexceeds a certain critical value. "Melt Fracture" is a general term usedby the polymer industry to describe a variety of extrudateirregularities observed during extrusion of molten polymers. Theoccurrence of melt fracture severely limits the rate at which acceptableproducts can be fabricated under commercial conditions. The occurrenceof melt fracture was first described by Nason in 1945 and since thenseveral investigators have studied this in an attempt to understand theunderlying mechanism(s) for its occurrence. C. J. S. Petrie and M. M.Denn (Amer. Inst. Chem. Engrs. Journal, Vol. 22, pages 209-236, 1976)have presented a critical review of the literature which indicates thatthe present understanding of the mechanism(s) leading to melt fractureis far from complete.

The melt fracture characteristics of a molten polymer is usually studiedusing a capillary rheometer. The polymer at a given temperature isforced through a capillary die of known dimensions at a given flow rate.The pressure required is noted and the emerging extrudate is examinedfor surface characteristics.

The extrudate surface characteristics of linear low density polyethylene(LLDPE) resins, as determined with a capillary rheometer are typical ofmany linear narrow distribution polymers. They indicate that at lowshear stresses (less than approximately 20 psi), the emerging extrudatesfrom a capillary die are smooth and glossy. At a critical shear stress(approximately 20-22 psi), the extrudates exhibit loss of surface gloss.The loss of gloss is due to fine scale roughness of the extrudatesurface which can be perceived under a microscope at a moderatemagnification (20-40×). This condition represents the "onset" of surfaceirregularities and occurs at a critical shear stress in the die. Abovethe critical stress, two main types of extrudate melt fracture can beidentified with LLDPE resins: surface melt fracture and gross meltfracture. The surface melt fracture occurs over a shear stress range ofapproximately 20-65 psi and results in increasing severity of surfaceroughness. In its most severe form, it appears as "sharkskin". Thesurface irregularities occur under apparently steady flow conditions.That is, no fluctuation in either the pressure or the flow rate isobserved with time. At a shear stress of approximately 65 psi, the flowbecomes unsteady when both the pressure and the flow rate fluctuatebetween two extremes and the emerging extrudates correspondingly exhibitsmooth and rough surfaces. This is the onset of gross melt fracture andhas been a subject of intense investigation by several investigatorsbecause of its severity. With further increase in shear stress, theextrudates become totally distorted and show no regularity.

Several mechanisms have been proposed in the literature for theoccurrence of surface and gross melt fracture. Surface melt fracture ofthe sharkskin type has been proposed to be due to effects at the dieexit where the viscoelastic melt is subjected to high local stresses asit parts company with the die surface. This leads to cyclic build up andrelease of surface tensile forces at the die exit resulting in theobserved surface melt fracture. Another mechanism for surface meltfracture proposes differential recovery, due to the elasticity betweenthe skin and core of the extrudate as a primary cause. On the otherhand, gross metal fracture has been proposed to be due to die landand/or die entry effects. The proposed mechanisms include: "slip-stick"in the die land region; tearing of the melt in the die entry region dueto exceeding the melt strength; and, propagation of spiralling flowinstabilities in the die entry region.

Under commercial film fabrication conditions (shear stress rangeapproximately 25-65 psi) with conventional blown films dies,predominantly surface melt fracture of the sharkskin type occurs withLLDPE resins resulting in commercially unacceptable products.

There are several methods for eliminating surface melt fracture undercommercial film fabrication conditions. These are aimed at reducing theshear stresses in the die and include: increasing the melt temperature;modifying the die geometry; and use of slip additives in the resin toreduce friction at the wall. Increasing the melt temperature is notcommercially useful since it lowers the rate for film formation due tobubble instabilities and heat transfer limitations. Another method foreliminating sharkskin is described in U.S. Pat. No. 3,920,782. In thismethod, surface melt fracture formed during extrusion of polymericmaterials is controlled or eliminated by cooling an outer layer of thematerial, so it emerges from the die with a reduced temperature whilemaintaining the bulk of the melt at the optimum working temperature.However, this method is difficult to employ and control.

The invention of U.S. Pat. No. 3,920,782 is apparently based on theinventor's conclusions that the onset of surface melt fracture under hisoperating conditions with his resins is a function, basically, ofexceeding a critical linear velocity with his resins through his dies athis operating temperatures. In the process of the present invention,however, the onset of surface melt fracture in the present applicant'sresins under his operating conditions is a function, primarily ofexceeding a critical shear stress.

U.S. Pat. No. 3,382,535 discloses a means for designing dies which areto be used for the high speed extrusion coating of wire and cable withplastic materials such as polypropylene, high density and low densitypolyethylene together with their copolymers, which are responsive, orsensitive, to the taper angles of the extrusion die. The dies of thispatent are designed to avoid gross melt fracture of the extruded plasticwire coating which is encountered at significantly higher stresses thanthat for surface melt fracture encountered during film formation.

The invention of U.S. Pat. No. 3,382,535 resides in the designing of thetaper angle of the die entry so as to provide a curvilinear dieconfiguration (FIGS. 6 and 7 of the patent) which converges in thedirection of flow of the resin. This procedure however, of, in effect,decreasing the taper angle of the die, will result in an increase in thecritical shear rate of the resin processed through the die. This reducesgross distortions as a function only of the angle of entry in and/or tothe die. Surface melt fracture is insensitive to taper angles at the dieentry and the present invention relates to reducing surface meltfracture as a function of the materials of construction of the die landregion including the die exit whereby significantly higher shear ratescan be obtained without encountering surface melt fracture during filmfabrication.

U.S. Pat. No. 3,879,507 discloses a means of reducing melt fractureduring the extrusion of foamable composition into film or sheet. Thismethod involves increasing the length of the die land and/or slightlytapering the die gap, while retaining or decreasing the die gap, whichis apparently to be relatively narrow, as compared to the prior art (seecolumn 4, lines 2-6) and of the order of 0.025 inches or 25 mils (column5, line 10). This kind of melt fracture is produced by premature bubbleformation at the surface. This melt fracture, however, is totallydifferent than the melt fracture experienced in processing LLDPE resinsfor film formation. In other words, the melt fracture is not as a resultof rheological properties as discussed herein. Die modifications aredesigned to reduce the shear stress in the die land region to be belowthe critical stress level (approximately 20 psi) by either enlarging thedie gap (U.S. Pat. Nos. 4,243,619 and 4,282,177) or heating the die lipto temperatures significantly above the melt temperature. Enlarging thedie gap results in thick extrudates which must be drawn down and cooledin the film blowing process. While LLDPE resins have excellent drawdowncharacteristics, thick extrudates increase the molecular orientation inthe machine direction and results in directional imbalance and reductionin critical film properties such as tear resistance. Also, thickextrudates limit the efficiency of conventional bubble cooling systemswhich result in reduced rates for stable operation. The wide gaptechnology has other disadvantages. The required gap is a function ofextrusion rate, resin melt index and melt temperature. The wide gapconfiguration is not suitable for processing conventional Low DensityPolyethylene (HP-LDPE) resins. Thus, die gap changes are required toaccommodate the flexibility expected by the fabricator with a givenline.

The heated lip concept is aimed at reducing stresses at the die exit andinvolves extensive modifications requiring efficient insulation of thehot lips from the rest of the die and from the air ring.

U.S. Pat. No. 3,125,547 discloses a polyolefin composition involving theaddition of a fluorocarbon polymer to provide improved extrusioncharacteristics and melt fracture free extrudates at high extrusionspeeds. This is based on the inventor's conclusion that the slip-stickphenomenon at high extrusion speeds and the resulting herring bonepattern on the extrudate surface are due to poor lubrication at the dieorifice. The use of the fluorocarbon polymer is intended to promotelubrication and reduce the stresses involved to obtain melt fracturefree extrudates. The present invention, however, is based on an exactlyopposite reasoning in that, it is the lack of adhesion, rather than lackof lubrication, at the polymer/metal interface in the die land region asthe cause of both surface and gross metal fracture in LLDPE resins. Thepresent invention, thus aims at improving the adhesion at the interfaceby proper choice of the material of construction of the die land region,including the die exit, to achieve melt fracture free extrudates. Thepractice of U.S. Pat. No. 3,125,547 drastically reduces the stresseswith dies constructed from conventional materials which, apparentlysuggests a modification of the rheological properties of the polyolefinresin due to the presence of the fluorocarbon polymer. The process ofthe present invention, involving a different material of constructionfor the die land region, achieves melt fracture free extrudates withoutsignificantly affecting the stresses involved or the rheologicalproperties of the resin.

U.S. Pat. No. 4,342,848 discloses the use of Polyvinyloctadecyl Ether asa processing modifier to obtain smoother surface of the extrudates andimproved film properties with high density polyetheylene resins. Thisadditive, however, was found to be unsuitable for melt fracturereductions with LLDPE resins.

Additives for use as processing aids to obtain melt fracture reductionin extrudates, are expensive and the added cost, depending on therequired concentration, may be prohibitive in resins, such as granularLLDPE, intended for commodity applications. Additives influence therheological properties of the base resin and, in excess amounts, mayadversely affect critical film properties including gloss, transparency,blocking and heat sealability characteristics of the product.

In the process of the present invention, surface melt fracture, can bevirtually eliminated by changes in the die i.e., by extruding the narrowmolecular weight distribution ethylene polymer at normal film extrusiontemperatures through a die having a die land region defining opposingdie exit surfaces and wherein at least one, and preferably both of theopposing die exit surfaces are fabricated from an alloy containing 5 to95 parts by weight zinc and 95 to 5 parts by weight of copper to provideat least one and preferably two zinc/copper alloy surfaces in contactwith the molten polymer. The utility of the present invention arises asa result of the discovery that the primary mechanism for the onset ofsurface melt fracture in LLDPE resins is the initiation of slip ofpolymer melt at the die wall. Slip is due to the breakdown of adhesionat the polymer/metal interface under flowing conditions and occurs at acritical shear stress. Adhesion is a surface phenomenon being stronglydependent on the nature of surfaces and the intimacy of contact ofsurfaces. Thus, techniques to provide good adhesion at the flowingpolymer/die wall interface will result in the elimination of surfacemelt fracture for LLDPE resins. Improvements in adhesion can be achievedby either proper choice of materials of construction of the die for agiven resin or use of adhesion promoters in the resin for a givenmaterial or a proper combination of both. In the present invention, ithas been found that use of a zinc/copper alloy surface for the die landregion eliminates surface melt fracture during film fabrication of LLDPEresins with narrow die gaps at commercial rates.

In the case where only one surface of the opposing die land surfaces isconstructed from the zinc/copper alloy, then surface melt fracture isreduced or eliminated on the surface of the polymer adjacent to thezinc/copper alloy. If both surfaces of the opposing die land areconstructed from the alloy, then both surfaces of the polymer would havereduced melt fracture.

Films suitable for packaging applications must possess a balance of keyproperties for broad end-use utility and wide commercial acceptance.These properties include film optical quality, for example, haze, gloss,and see-through characteristics. Mechanical strength properties such aspuncture resistance, tensile strength, impact strength, stiffness, andtear resistance are important. Vapor transmission and gas permeabilitycharacteristics are important considerations in perishable goodspackaging. Performance in film converting and packaging equipment isinfluenced by film properties such as coefficient of friction, blocking,heat sealability and flex resistance. Low density polyethylene has awide range of utility such as in food packaging and non-food packagingapplications. Bags commonly produced from low density polyethyleneinclude shipping sacks, textile bags, laundry and dry cleaning bags andtrash bags. Low density polyethylene film can be used as drum liners fora number of liquid and solid chemicals and as protective wrap insidewooden crates. Low density polyethylene film can be used in a variety ofagricultural and horticultural applications such as protecting plantsand crops, as mulching, for storing of fruits and vegetables.Additionally, low density polyethylene film can be used in buildingapplications such as a moisture or moisture vapor barrier. Further, lowdensity polyethylene film can be coated and printed for use innewspapers, books, etc.

Possessing a unique combination of the aforedescribed properties, highpressure low density polyethylene is the most important of thethermoplastic packaging films. It accounts for about 50% of the totalusage of such films in packaging. Films made from the polymers of thepresent invention, preferably the ethylene hydrocarbon copolymers, offeran improved combination of end-use properties and are especially suitedfor many of the applications already served by high pressure low densitypolyethylene.

An improvement in any one of the properties of a film such aselimination or reduction of surface melt fracture or an improvement inthe extrusion characteristics of the resin or an improvement in the filmextrusion process itself is of the utmost importance regarding theacceptance of the film as a substitute for high pressure low densitypolyethylene in many end use applications.

DRAWINGS

FIG. 1 shows a cross section of a spiral/spider annulus die.

FIG. 2 shows an enlarged cross section of a portion of a spiral die.

FIG. 3 shows a configuration of the die land region wherein the opposingzinc/copper alloy surfaces are provided by zinc/copper alloy inserts.

FIG. 4 shows a configuration of the die land region wherein the opposingzinc/copper alloy surfaces are provided by solid zinc/copper alloyconstruction of the zinc/copper alloy collar and zinc/copper alloy pin.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a processfor substantially eliminating surface melt fracture during extrusion ofa molten narrow molecular weight distribution, linear, ethylenecopolymer, under conditions of flow rate and melt temperature whichwould otherwise produce such surface melt fracture which comprisesextruding said polymer through a die having a die land region definingopposing surfaces, and wherein at least one of said opposing surface isfabricated from an alloy containing 5 to 95 parts by weight zinc and 95to 5 parts by weight of copper whereby melt fracture is substantiallyeliminated on the surface of the polymer adjacent to said zinc/coppercontaining alloy surface.

Preferably both opposing surfaces contain the zinc/copper alloy adjacentthe polymer.

In a broader aspect of the invention, it has now been determined that inaddition to the ethylene polymers recited therein, that the practice ofthe present invention is also applicable to thermoplastic polymers whichexperience surface melt fracture during extrusion at flow rates and melttemperatures which produce such melt fracture. Examples of thermoplasticmaterials in which surface melt fracture can be observed includepolypropylene, polystyrene, styrene-butadiene-styrene,polyvinylchloride, polyacrylonitrile, and like polymers. Thus, in abroader aspect the present invention provides a process forsubstantially eliminating surface melt fracture during extrusion of athermoplastic polymer under conditions of flow rate and melt temperaturewhich would otherwise produce such melt fracture which comprisesextruding said thermoplastic polymer through a die having a die landregion defining opposing surfaces, and wherein at least one of saidopposing surface is fabricated from an alloy containing 5 to 95 parts byweight zinc and 95 to 5 parts by weight of copper whereby melt fractureis substantially eliminated on the surface of the polymer adjacent tosaid zinc/copper containing alloy surface.

In addition in a preferred operative mode, it is desirable to minimizemoisture content in the resin entering the extruder. This can beaccomplished by hopper dryers, or use of an inert gas such as nitrogenin the hopper or preferably at the throat of the extruder or hopper.

The present invention also provides a die for extruding thermoplasticpolymers prone to melt fracture which comprises a die block, havinginlet means for introducing an extruder melt into said die block andoutlet means for discharging molten polymer from said die block, conduitmeans for passing molten material from said inlet means to said outletmeans said outlet means including a land entry section and a die land,said die land having surfaces in contact with said molten polymer, atleast one of said surfaces being fabricated from an alloy containing 5to 95 parts by weight zinc and 95 to 5 parts by weight of copper wherebymelt fracture is substantially eliminated on the surfaces of the polymeradjacent to said zinc/copper containing alloy surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT Dies

Advantageously, the molten ethylene polymer can be extruded through adie such as a spiral annular die, slit die, etc., preferably an annulardie, having a narrow die gap up to about 50 mils and preferably about5-40 mils. Advantageously, when processing LLDPE resins, it is no longerrequired to extrude the molten ethylene polymer through a die having adie gap of greater than about 50 mils to less than about 120 mils, asdescribed in U.S. Pat. No. 4,243,619. Conventionally, die land regionconstruction has been largely based on nickel or chrome plated steelsurfaces.

FIG. 1 is a cross-sectional view of a spiral/spider annular die 10through which the molten thermoplastic ethylene polymer is extruded toform a single layer film, tube or pipe. The solid thermoplastic polymere.g., ethylene polymer is introduced into the extruder from hopper 11together with an inert fluid introduced at the throat of the extruder 13and intended to provide a dry atmosphere. The preferred fluid isnitrogen gas at a rate of about 1 to 3 standard cubic feet per hour. Dieblock 12 contains channels 14 for directing the polymer to the die exit.As the molten thermoplastic ethylene polymer is extruded, its spreadsout as it passes into the die channels 14.

Referring to FIG. 2, which is a cross-section of a spiral die, there isindicated a spiral section J land entry section H and die land G. Withreference to FIGS. 1 and 2, at the exit of the die, there is a diedischarge outlet identified generally by reference numeral 16. Thedischarge outlet defines an exit die gap 18 which is formed by opposingsurfaces of die lips 20 and 20' extending from opposing die landsurfaces 22 and 22'.

As shown in FIGS. 3 and 4, the die land region shows a configurationwherein opposing surfaces are fabricated from an alloy containingzinc/copper material as contrasted to conventional nickel or chromeplated steel. The surfaces can be provided by inserts 24 which aresecured, preferably detachably secured to the pin and the collar. Theinserts can be detachably secured to the modified pin and collar by anysuitable means such as by provision of threaded elements disposedinteriorly of the inserts which threadably engage in a matingrelationship threaded elements of the corresponding surface of the pinor collar. The length measured in the direction of extrudate flow, ofthe inserts are preferably the length of the die land region althoughshorter lengths are operable. Other techniques for providing thezinc/copper containing alloy surface can be utilized such as by coatingthe surfaces of the die land region with the alloy or alternatively byfabricating either the die land section or the entire pin and collarfrom the alloy as shown in FIG. 4.

The melt fracture is reduced on the surface of the polymer adjacent tothe alloy surface. As a result, the process can be practiced with theinvention disclosed in U.S. Pat. No. 4,348,349 issued on Sept. 7, 1982.Advantageously, therefore, melt fracture can be reduced on both sides ofa film by directing the molten polymer through the die land regionwherein only the surface of film in which melt fracture is to be reducedor eliminated is adjacent to the zinc/copper alloy surface and on theother surface melt fracture would also be eliminated as disclosed in thepatent. Also, according to the present invention, processing ofmulti-layer films is also possible wherein one layer is formed of LLDPEand another layer is formed from a resin which under the conditions ofoperation is not subject to melt fracture. Thus, by the process of theinstant invention, the LLDPE resin can be passed through the die incontact with the zinc alloy surface whereas the resin not subject tomelt fracture is extruded in contact with the other die land surfacethereby producing a multi-layer film, both outer surfaces of which wouldbe free of melt fracture.

As mentioned previously, the surface of the die land region adjacent tothe molten polymer is constructed from an alloy containing zinc andcopper. Various other types of metal surfaces are examined in an attemptto reduce/eliminate surface melt fracture. These surfaces included:conventional chrome plated steel; titanium nitride plated steel; purecopper; pure zinc; Berryllium copper; carbon steel (4140); and, nickelsteel (4340). The detailed extrudate surface characteristics atcomparable film fabrication conditions were found to be different witheach of the above surfaces but none were found to be effective inreducing surface melt fracture to commercially acceptable levels.

The zinc/copper containing alloy was found to be particularly suitablewhen used in amounts containing 5 to 95 parts by weight zinc and 95 to 5parts by weight of copper.

Preferred amounts are 30 to 40 parts by weight zinc and 70 to 60 partsby weight copper. Superior results are obtained when the zinc/coppercontaining alloy commercially available as naval brass or free-cuttingbrass, is utilized. The zinc/copper containing alloy can contain otherelements such as tin, aluminum, lead etc.

With brass surfaces for the die land region, surface melt fractureappears initially during startup which it is believed is due to thepresence of absorbed oxide film. Following a brief induction period,which depends on the rate of extrusion, the extrudate becomes free ofsurface melt fracture and remains so for an interval depending on theextrusion rate. Surface melt fracture reappears after this interval.This is believed to be due to the degradation of the brass surface as aresult of dezincification of brass, at temperatures employed forprocessing LLDPE resins, thus affecting the adhesion characteristics atthe polymer/brass interface. It has been found that the use of asuitable stabilizing additive in the resin eliminates this timedependency with brass surfaces. Thus, for prolonged operation it ispreferred that a suitable stabilizing additive be used which can beincluded in the masterbatch added to the copolymer. A suitablestabilizing additive for use with brass is fatty diethoxylated tertiaryamine, commercially available as Kemamine AS 990 from Witco ChemicalCorporation, Memphis, Tenn. Other conventional stabilizing additives canalso be utilized. Addition of 50-1500 ppm, but preferably in the rangeof 300-800 ppm, of the tertiary amine is effective in eliminating therecurrence of melt fracture with brass. This stabilizer can be includedin the Masterbatches conventionally used for providing requiredantiblock and slip characteristics for the product.

Film Extrusion

I. Blown Film Extrusion

The films formed as disclosed herein may be extruded by tubular blownfilm extrusion process. In this process a narrow molecular weightdistribution polymer is melt extruded through an extruder. This extrudermay have an extrusion screw therein with a length to diameter ratio ofbetween 15:1 to 21:1, as described in U.S. Pat. No. 4,343,755 in thenames of John C. Miller et al and entitled "A Process For ExtrudingEthylene Polymers". This application describes that this extrusion screwcontains a feed, transition and metering section. Optionally, theextrusion screw can contain a mixing section such as that described inU.S. Pat. Nos. 3,486,192; 3,730,492 and 3,756,574, which areincorporated herein by reference. Preferably, the mixing section isplaced at the screw tip.

The extruder which may be used herein may have a 18:1 to 32:1 length tointernal diameter barrel ratio. The extrusion screw used in the presentinvention may have a length to diameter ratio of 15:1 to 32:1. When, forexample, an extrusion screw of a length to diameter ratio of 18:1 isused in a 24:1 extruder, the remaining space in the extrusion barrel canbe partially filled with various types of plugs, torpedoes, or staticmixers to reduce residence time of the polymer melt.

The extrusion screw can also be of the type described in U.S. Pat. No.4,329,313. The molten polymer is then extruded through a die, as willhereinafter be described.

The polymer is extruded at a temperature of about 163° C. to about 260°C. The polymer is extruded in an upward vertical direction in the formof a tube although it can be extruded downward or even sideways. Afterextrusion of the molten polymer through the annular die, the tubularfilm is expanded to the desired extent, cooled, or allowed to cool andflattened. The tubular film is flattened by passing the film through acollapsing frame and a set of nip rolls. These nip rolls are driven,thereby providing means for withdrawing the tubular film away from theannular die.

A positive pressure of gas, for example, air or nitrogen, is maintainedinside the tubular bubble. As is known in the operation of conventionalfilm processes, the pressure of the gas is controlled to give thedesired degree of expansion to the tubular film. The degree ofexpansion, as measured by the ratio of the fully expanded tubecircumference to the circumference of the die annulus, is in the range1:1 to 6:1 and preferably, 1:1 to 4:1. The tubular extrudate is cooledby conventional techniques such as, by air cooling, water quench ormandrel.

The drawdown characteristics of the polymers disclosed herein areexcellent. Drawdown, defined as the ratio of the die gap to the productof film gauge and blow up ratio, is kept less than about 250. Very thingauge films can be produced at high drawdown from these polymers evenwhen said polymer is highly contaminated with foreign particles and/orgel. Thin gauge films of about 0.5 to 3.0 mils can be processed toexhibit ultimate elongations MD greater than about 400% to about 700%and TD greater than about 500% to about 700%. Furthermore, these filmsare not perceived as "splitty". "Splittiness" is a qualitative termwhich describes the notched tear response of a film at high deformationrates. "Splittiness" reflects crack propagation rate. It is an end-usecharacteristic of certain types of film and is not well understood froma fundamental perspective.

As the polymer exits the annular die, the extrudate cools and itstemperature falls below its melting point and it solidifies. The opticalproperties of the extrudate change as crystallization occurs and a frostline is formed. The position of this frost line above the annular die isa measure of the cooling rate of the film. This cooling rate has a verymarked effect on the optical properties of the film produced herein.

The ethylene polymer can also be extruded in the shape of a rod or othersolid cross section using the same die geometry for only the externalsurface. Additionally, the ethylene polymer can also be extruded intopipe through annular dies.

II. Slot Cast Film Extrusion

The films formed as disclosed herein may also be extruded by slot castfilm extrusion. This film extrusion method is well known in the art andcomprises extruding a sheet of molten polymer through a slot die andthen quenching the extrudate using, for example, a chilled casting rollor water bath. The die will hereinafter be described. In the chill rollprocess, film may be extruded horizontally and laid on top of the chillroll or it may be extruded downward and drawn under the chill roll.Extrudate cooling rates in the slot cast process are very high. Chillroll of water bath quenching is so fast that as the extrudate coolsbelow its melting point, crystallites nucleate very rapidly,supramolecular structures have little time to grow and spherulites areheld to a very small size. The optical properties of slot cast film arevastly improved over those characterizing films using the slower coolingrate, tubular blown film extrusion process. Compound temperatures in theslot cast film extrusion process generally run much higher than thosetypifying the tubular blown film process. Melt strength is not a processlimitation in this film extrusion method. Both shear viscosity andextensional viscosity are lowered. Film can generally be extruded athigher output rates than practiced in the blown film process. The highertemperatures reduce shear stresses in the die and raise the outputthreshold for surface melt fracture.

Film

The film produced by the method of the present invention has a thicknessof greater than about 0.10 mils to about 20 mils, preferably greaterthan about 0.10 to 10 mils, most preferably greater than about 0.10 to4.0 mils. The 0.10 to 4.0 mil film is characterized by the followingproperties: a puncture resistance value of greater than about 7.0in-lbs/mil; an ultimate elongation of greater than about 400%, tensileimpact strength of greater than about 500 to about 2000 ft-lbs/in³ andtensile strength greater than about 2000 to about 7000 psi.

Various conventional additives such as slip agents, antiblocking agents,and antioxidants can be incorporated in the film in accordance withconventional practice.

The Ethylene Polymers

The polymers which may be used in the process of the present inventionare homopolymers of ethylene or copolymers of a major mol percent(greater than or equal to 80%) of ethylene, and a minor mol percent(less than or equal to 20%) of one or more C₃ to C₈ alpha olefins. TheC₃ to C₈ alpha olefins should not contain any branching on any of theircarbon atoms which is closer than the fourth carbon atom. The preferredC₃ to C₈ alpha olefins are propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1 and octene-1.

The ethylene polymers have a melt flow ratio of about greater than orequal to 18 to less than or equal to 50, and preferably of about greaterthan or equal to 22 to less than or equal to 30.

The homopolymers have a density of about greater than or equal to 0.958to less than or equal to 0.972 and preferably of about greater than orequal to 0.961 to less than or equal to 0.968. The copolymers have adensity of about greater than or equal to 0.89 to less than or equal to0.96 and preferably greater than or equal to 0.917 to less than or equalto 0.955, and most preferably, of about greater than or equal to 0.917to less than or equal to 0.935. The density of the copolymer, at a givenmelt index level for the copolymer, is primarily regulated by the amountof the C₃ to C₈ comonomer which is copolymerized with the ethylene. Inthe absence of the comonomer, the ethylene would homopolymerize with thecatalyst of the present invention to provide homopolymers having adensity of about greater than or equal to 0.96. Thus, the addition ofprogressively larger amounts of the comonomers to the copolymers resultsin a progressive lowering of the density of the copolymer. The amount ofeach of the various C₃ to C₈ comonomers needed to achieve the sameresult will vary from monomer to monomer, under the same reactionconditions.

When made in the fluid bed process, polymers of the present inventionare granular materials which have a settled bulk density of about 15 to32 pounds per cubic foot and an average particle size of the order ofabout 0.005 to about 0.06 inches.

For the purposes of making film in the process of the present invention,the preferred polymers are the copolymers and particularly thosecopolymers having a density of about greater than or equal to 0.917 toless than or equal to 0.924; and a standard melt index of greater thanor equal to 0.1 to less than or equal to 5.0.

The Propylene Polymers

The propylene polymers which can be used in the process of the presentinvention, span a melt flow range (measured according to ASTM-1238Condition L) from about 0.1 to 100 g/10 min. and are from amongpolypropylene homopolymers with an isotatic index (insoluble with refluxin heptane over a 24 hour period) greater than or equal to 85%, randomcopolymers with a total major molar percent of greater than or equal to85% of propylene and a minor molar percent of less than or equal to 15%of ethylene and/or one or more of C₃ to C₈ alpha olefins and block (orimpact copolymers) with a total major weight percent from about 70 toabout 95% of propylene and a minor weight percent from about 5 to about30% of ethylene and/or one or more of C₃ to C₈ alpha olefins.

The process of the present invention is also applicable to extrusion ofthermoplastic resins for other than film making purposes. Thus inaddition to reduction or elimination of surface melt fracture fromfilms, the process can also be used to reduce or eliminate surface meltfracture in thin wall tubes, pipes etc. which would otherwise besusceptible to surface melt fracture under conventional surface meltfracture conditions.

The films made in the process of the present invention have a thicknessof greater than 0.1 mil to less than or equal to 10 mils and preferablyof greater than 0.1 mil to less than or equal to 5 mils.

Having set forth the general nature of the invention, the followingexamples illustrate some specific embodiments of the invention. It is tobe understood, however, that this invention is not limited to theexamples, since the invention may be practiced by the use of variousmodifications.

EXAMPLE 1

This Example demonstrates a conventional procedure (except for thenitrogen purge in the hopper) for extruding ethylene polymers intotubes.

An ethylene-butene copolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is available from UnionCarbide Corporation under the Trademark designation Bakelite GRSN 7047.The copolymer was dry blended with 4% of a conventional masterbatchcontaining conventional antiblock agent, slip agent and antioxidants andalso included 320 ppm by weight of Kemamine AS 990. The copolymer had anominal density of 0.918 gm/cc, a nominal melt index of 1.0decigrams/min., and a nominal melt flow ratio of 26. The copolymer wasformed into a tube by passing the resin from a hopper which was purgedwith nitrogen at 3 standard cubic feet per hour through a conventional21/2 inch diameter screw extruder having a polyethylene screw asdescribed in U.S. Pat. No. 4,329,313 with a Maddock mixing section, andthence into a conventional chrome plated die having a 0.5 inch land, 3inch die collar diameter and a die pin diameter normally of 2.92 inchesto give a 40 mil. die gap. The sides of the die land were parallel withthe flow axis of the polymer melt. The resin was extruded through thedie at a rate of 66 lbs/hr at a temperature of 221° C. There was severesurface melt fracture observed on both surfaces of the tube.

EXAMPLE 2

This Example demonstrates a conventional procedure for extruding adifferent type of ethylene polymer into tubes.

The ethylene-butene copolymer was prepared in accordance with theprocedures of U.S. Pat. No. 4,302,566 and which is available from UnionCarbide Corporation under the Trademark designation Bakelite GERS-6937.The copolymer also contained 4% of the masterbatch, as in Example 1. Thecopolymer has a nominal density of 0.918 gm/cc, a nominal melt index of0.5 decigrams/minute and a nominal melt flow ratio of 26. The copolymerwas formed into a tube by passing the resin after drying with nitrogenthrough the conventional 21/2 inch diameter extruder and mixer ofExample 1 and into the die of Example 1. The resin was extruded throughthe die at a rate of 68 lbs/hr and at a temperature of 229° C. There wassevere surface melt fracture observed on both surfaces of the tube.

EXAMPLE 32

This Example demonstrates a conventional procedure for extruding anotherethylene polymer into tubes.

An ethylene-butene-hexene terpolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is produced by UnionCarbide Corporation under the designation DEX-7652. The copolymer alsocontained 4% of the masterbatch, as in Example 1. The copolymer had anominal density of 0.918 gm/cc, a nominal melt index of 1.0decigrams/minute and a nominal melt flow ratio of 26. The copolymer wasformed into a tube by passing the resin after drying with nitrogenthrough a conventional 21/2 inch diameter extruder and mixer of Example1 and into the die of Example 1. The resin was extruded through the dieat a rate of 68 lbs/hr and at a temperature of 220° C. There was severesurface melt fracture observed on both surfaces of the tube.

EXAMPLE 4

This Example demonstrates a conventional procedure for extruding anotherethylene polymer into tubes.

An ethylene-butene-hexene terpolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is produced by UnionCarbide Corporation under designation DEX-7653. The copolymer alsocontained 4% of the masterbatch, as in Example 1. The copolymer had anominal density of 0.918 gm/cc, a nominal melt index of 0.5decigrams/minute and a nominal melt flow ratio of 26. The copolymer wasformed into a tube by passing the resin after drying with nitrogenthrough a conventional 21/2 inch diameter extruder and mixer of Example1 and into the die of Example 1. The resin was extruded through the dieat a rate of 68 lbs/hr and at a temperature of 229° C. There was severesurface melt fracture observed on both surfaces of the tube.

EXAMPLE 5

This Example demonstrates the improved reesults obtained over Example 1by the use of free cutting Brass die pin and collar nominally containing35% Zinc, 61.5% Copper, 3% lead and 0.5% iron.

The ethylene-butene copolymer was identical to Example 1 and containedthe masterbatch. The copolymer was formed into a tube by passing theresin after drying with nitrogen fed at a rate of 3 standard cubic feetper hour through the conventional 21/2 inch diameter extruder and mixerof Example 1 and into the die of Example 1 except for the brass surfaceof the die land. The resin was extruded through the die at a rate of 66lbs/hr and at a temperature of 220° C. Other than during the initialstart-up (induction period) there was no surface melt fracture on eithersurface of the tube.

EXAMPLE 6

This Example demonstrates the improved results obtained over Example 2by the use of free cutting Brass die pin and collar nominally containing35% Zinc, 61.5% Copper, 3% lead and 0.5% iron.

The ethylene-butene copolymer was identical to Example 2 and containedthe masterbatch. The copolymer was formed into a tube by passing theresin after drying with nitrogen through the conventional 21/2 inchdiameter extruder and mixer of Example 1 and into the die of Example 1except for the brass surface of the die land. The resin was extrudedthrough the die at a rate of 68 lbs/hr and at a temperature of 229° C.Other than during the initial start-up (induction period) there was nosurface melt fracture on either surface of the tube.

EXAMPLE 7

This Example demonstrates the improved results obtained over Example 3by the use of free cutting Brass die pin and collar nominally containing35% Zinc, 61.5% Copper, 3% lead and 0.5% iron.

The ethylene-butene-hexene terpolymer was identical to Example 3 andcontained the masterbatch. The terpolymer was formed into a tube bypassing the resin after drying with nitrogen through the conventional21/2 inch diameter extruder and mixer of Example 1 and into the die ofExample 1 except for the brass surface of the die land. The resin wasextruded through the die at a rate of 68 lbs/hr and at a temperature of220° C. Other than during the initial start-up (induction period) therewas no surface melt fracture on either surface of the tube.

EXAMPLE 8

This Example demonstrates the improved results obtained over Example 4by the use of free cutting Brass die pin and collar nominally containing35% Zinc, 61.5% Copper, 3% lead and 0.5% iron.

The ethylene-butene-hexene terpolymer was identical to Example 4 andcontained the masterbatch. The terpolymer was formed into a tube bypassing the resin after drying with nitrogen through the conventional21/2 inch diameter extruder and mixer of Example 1 and into the die ofExample 1 except for the brass surface of the die land. The resin wasextruded through the die at a rate of 68 lbs/hr and at a temperature of229° C. Other than during the initial start-up (induction period) therewas no surface melt fracture on either surface of the tube.

EXAMPLE 9

This Example demonstrates the improved results over Example 1 by the useof free cutting Brass die pin and collar nominally containing 35% Zinc,61.5% Copper, 3% lead and 0.5% iron and a decreased die gap of 20 mil.

The ethylene-butene copolymer was identical to Example 1 and containedthe masterbatch. The copolymer was formed into a tube by passing theresin after drying with nitrogen through the conventional 21/2 inchdiameter meter extruder and mixer of Example 1 and into a die having adie gap of 20 mils. Other features of the die are as in Example 1. Thesides of the die land were parallel with the flow axis of the polymermelt. The resin was extruded at a rate of 66 lbs/hr and at a temperatureof 220° C. Following the induction period, there was no surface meltfracture observed on either surface of the tube.

EXAMPLE 10

This Example demonstrates the improved results over Example 2 by the useof free cutting Brass die pin and collar nominally containing 35% Zinc,61.5% Copper, 3% lead and 0.5% iron and a decreased die gap of 20 mil.

The ethylene-butene copolymer was identical to Example 2 and containedthe masterbatch. The copolymer was formed into a tube by passing theresin after drying with nitrogen through the conventional 21/2 inchdiameter meter extruder and mixer of Example 1 and into a die having adie gap of 20 mils. Other features of the die are as in Example 1. Thesides of the die land were parallel with the flow axis of the polymermelt. The resin was extruded at a rate of 68 lbs/hr and at a temperatureof 229° C. Following the induction period, there was no surface meltfracture observed on either surface of the tube.

EXAMPLE 11

This Example demonstrates the improved results over Example 3 by the useof free cutting Brass die pin and collar nominally containing 35% Zinc,61.5% Copper, 3% lead and 0.5% iron and a decreased die gap of 20 mil.

The ethylene-butene-hexene terpolymer was identical to Example 3 andcontained the masterbatch. The terpolymer was formed into a tube bypassing the resin after drying with nitrogen through the conventional21/2 inch diameter meter extruder and mixer of Example 1 and into a diehaving a die gap of 20 mils. Other features of the die are as inExample 1. The sides of the die land were parallel with the flow axis ofthe polymer melt. The resin was extruded at a rate of 68 lbs/hr and at atemperature of 220° C. Following the induction period, there was nosurface melt fracture observed on either surface of the tube.

EXAMPLE 12

This Example demonstrates the improved results obtained over Example 4by the use of free cutting Brass die pin and collar nominally containing35% Zinc, 61.5% Copper, 3% lead and 0.5% iron. Theethylene-butene-hexene terpolymer was identical to Example 4 andcontained the masterbatch. The terpolymer was formed into a tube bypassing the resin after drying with nitrogen through the conventional21/2 inch diameter extruder and mixer of Example 1 and into a die havinga die gap of 20 mils. The resin was extruded through the die at a rateof 68 lbs/hr and at a temperature of 229° C. Following the inductionperiod, there was no surface melt fracture on either surface of thetube.

EXAMPLE 13

This Example demonstrates the improved results over Example 1 by the useof free cutting Brass die pin and collar containing 35% Zinc, 61.5%Copper, 3% lead and 0.5% iron and a reduced die gap of 10 mil.

The ethylene-butene copolymer was identical to Example 1 and containedthe masterbatch. The copolymer was formed into a tube by passing theresin after drying with nitrogen through the conventional 21/2 inchdiameter extruder and mixer of Example 1 and into a die having a die gapof 10 mils and a 0.125 inch land length. The sides of the die land wereparallel with the flow axis of the polymer melt. The resin was extrudedat a rate of 66 lbs/hr and at a temperature of 221° C. Following theinduction period, there was no surface melt fracture observed on eithersurface of the tube.

EXAMPLE 14

This Example demonstrates the improved results over Example 1 by the useof free cutting Brass die pin and collar nominally containing 35% Zinc,61.5% Copper, 3% lead and 0.5% iron and a reduced die gap of 10 mil.

The ethylene-butene copolymer was identical to Example 1 and containedno masterbatch or the stabilizing agent (Kemamine AS990). The copolymerwas formed into a tube by passing the resin after drying with nitrogenthrough the conventional 21/2 inch diameter extruder and mixer ofExample 1 and into a die having a die gap of 10 mils and a 0.125 inchland length. The sides of the die land were parallel with the flow axisof the polymer melt. The resin was extruded at a rate of 66 lbs/hr andat a temperature of 222° C. There was no induction period and there wasno surface melt fracture observed on either surface of the tube.

EXAMPLE 15

This Example demonstrates the results for a different ethylene-butenecopolymer by the use of free cutting Brass die pin and collar nominallycontaining 35% Zinc, 61.5% Copper, 3% lead and 0.5% iron.

The ethylene-butene copolymer was prepared in accordance with theprocedure of U. S. Pat. No. 4,302,566 and which is produced by UnionCarbide Corporation under the designation Bakelite GRSN-7081. Thecopolymer is fully formulated with conventional antiblock agent, slipagent, and antioxidants and also contained 320 ppm by weight of KemamineAS 990. No additional masterbatch was used. The copolymer had a nominaldensity of 0.918 density, a nominal melt index of 1.0. decigrams/minuteand a nominal melt flow ratio of 26. The copolymer was formed into atube by passing the resin after drying with nitrogen through aconventional 21/2 inch diameter extruder and mixer of Example 1 and intoa die having a die gap of 20 mils as in Example 9. The resin wasextruded at a rate of 91 lbs/hr and at a temperature of 222° C.Following the brief induction period, there was no surface melt fractureobserved on either surface of the tube.

EXAMPLE 16

This Example demonstrates the results for another ethylene copolymer bythe use of free cutting Brass die pin and collar nominally containing35% Zinc, 61.5% Copper, 3% lead and 0.5% iron. This resin exhibitssevere sharkskin melt fracture when extruded through the conventionaldie of Example 1.

An ethylene-butene copolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is produced by UnionCarbide Corporation under the designation Bakelite GRSN-7071. Thecopolymer also contained 5% of a white concentrate in a masterbatch formidentified as MB-1900 available from South West Plastics Company. Inaddition, 800 ppm by weight of Kemamine AS 990 was dry blended with thecopolymer. The copolymer had a nominal density of 0.922 gm/cc, a nominalmelt index of 0.7 decigrams/minute and a nominal melt flow ratio of 26.The resin was formed into a tube by passing the resin after drying withnitrogen through a conventional 21/2 inch diameter extruder and mixer ofExample 1 and into a die having a die gap of 10 mils and a 0.125 inchland length as in Example 13. The resin was extruded at a rate of 70lbs/hr and at a temperature of 222° C. As before, there was no surfacemelt fracture observed on either surface of the tube.

EXAMPLE 17

This Examples demonstrates the results for yet another ethylenecopolymer by the use of free cutting Brass die pin and collar nominallycontaining 35% Zinc, 61.5% Copper, 3% lead and 0.5% iron. This resinexhibits severe sharkskin melt fracture when extruded through theconventional die of Example 1.

An ethylene-hexene copolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is produced by UnionCarbide Corporation under the designation Bakelite DEX-8218. Thecopolymer also contained 5% of a white concentrate in a masterbatch formidentified as MB-1900 available from South West Plastics Company. Inaddition, 800 ppm by weight of Kemamine AS 990 was dry blended with thecopolymer. The copolymer had a nominal density of 0.928 gm/cc, a nominalmelt index of 0.9 decigrams/minute and a nominal melt flow ratio of 26.The resin was formed into a tube by passing the resin after drying withnitrogen through a conventional 21/2 inch diameter extruder and mixer ofExample 1 and into a die having a die gap of 10 mils and a 0.125 inchland length as in Example 13. The resin was extruded at a rate of 70lbs/hr and at a temperature of 222° C. As before, there was no surfacemelt fracture observed on either surface of the tube.

EXAMPLE 18

This Example demonstrates the results with the offset configuration forthe chrome plated die as disclosed in U.S. Pat. No. 4,348,349 whereinthe surface melt fracture is reduced on the side of the tube in contactwith the die lip having a positive offset.

The ethylene-butene copolymer was identical to Example 1 and containedno masterbatch. Instead, 800 ppm by weight of Kemamine AS 990 was dryblended with the copolymer. The copolymer was formed into a tube bypassing the resin after drying with nitrogen through a conventional 21/2inch diameter extruder and mixer of Example 1 and into a conventional 3inch diameter die having a die gap of 40 mils and a land length of 1.375inches. The sides of the die land were parallel to the flow axis of thepolymer melt except that the top surface of the die pin was 120 milsabove that of the collar. The resin was extruded through the die at arate of 66 lbs/hr and at a temperature of 221° C. As disclosed in theU.S. Pat. No. 4,348,349 severe surface melt fracture was observed on theoutside of the tube with little or no surface melt fracture on theinside of the tube.

EXAMPLE 19

This Example demonstrates that results similar to the offsetconfiguration for the die can be obtained without offsetting the die lipbut by using free cutting Brass die pin nominally containing 35% Zinc,61.5% Copper, 3% lead and 0.5% iron and a conventional chrome plated diecollar.

The ethylene-butene copolymer was identical to Example 1 and containedno masterbatch. Instead, 800 ppm by weight of Kemamine AS 990 was dryblended with the copolymer. The copolymer was formed into a tube bypassing the resin after drying with nitrogen through a conventional 21/2inch diameter extruder and mixer of Example 1 and into a die having a 40mil die gap provided by a conventional die collar of 3 inch diameter anda brass die pin with a diameter of 2.92 inches. The die pin and thecollar were level and had no offset as in Example 17. Other features ofthe die are as in Example 17. The sides of the die land were parallel tothe flow axis of the polymer melt. The resin was extruded through thedie at a rate of 66 lbs/hr and at a temperature of 221° C. Following abrief induction period, surface melt fracture was observed on theoutside of the tube which was in contact with the conventional chromeplated die collar and no surface melt fracture on the inside of the tubewhich was adjacent to the brass surface.

EXAMPLE 20

This Example demonstrates the improved results over Example 18 by theuse of free cutting Brass die pin nominally containing 35% Zinc, 61.5%Copper, 3% lead and 0.5% iron and a conventional chrome plated diecollar with a positive offset.

The ethylene-butene copolymer was identical to Example 1 and containedno masterbatch. Instead, 800 ppm of the stabilizing agent (Kemamine AS990) was dry blended with the copolymer. The copolymer was formed into atube by passing the resin after drying with nitrogen through aconventional 21/2 inch diameter extruder and mixer of Example 1 and intothe die of Example 18 except for a 120 mil positive offset for theCollar. Other features of the die are as in Example 17. The sides of thedie land were parallel to the flow axis of the polymer melt. The resinwas extruded through the die at a rate of 67 lbs/hr and at a temperatureof 221° C. Following a brief induction period, very little surface meltfracture was observed on either surface of the tube.

EXAMPLE 21

This example demonstrates the performance of Free Cutting Brass surfacesfor the die land region without the use of Nitrogen purge at the throatof the extruder.

An ethylene-butene copolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is available from UnionCarbide Corporation under the Trademark designation Bakelite GRSN-7087.The copolymer contained 5% of a masterbatch identified as DFDC-0073available from Union Carbide Corporation. The masterbatch containedconventional antiblock agent, slip agent, antistatic agent (Kemamine AS990) and anti-oxidants. The copolymer had a nominal density of 0.918gm/cc, a nominal melt index of 1.0 decigrams/minute and a nominal meltflow ratio of 26. The copolymer was formed into a tube by passing theresin through a conventional 31/2 inch diameter screw extruder having apolyethylene screw as described in U.S. Pat. No. 4,329,313 with aMaddock mixing section and thence into a die having a 0.8 inch land, 8inches die collar and a die pin diameter of 7.92 inches to give a 40 mildie gap. The die collar and pin were constructed from Free CuttingBrass. The sides of the die land region were parallel with the flow axisof the polymer melt. The resin was extruded through the die at a rate of229 lbs/hr and at a temperature of 220 deg. C. Surface melt fracture wassubstantially reduced although there were several fine bands of surfaceimperfections observed on both sides of the tube.

EXAMPLE 22

This example demonstrates the improved results over Example 21 by theuse of Nitrogen at the throat of the extruder with the die having FreeCutting Brass surfaces for the mandrel extension region.

The ethylene-butene copolymer was identical to Example 21 and containedthe masterbatch. The copolymer was formed into a tube using the extruderand the 8 inch die, having the Free Cutting Brass surfaces for themandrel extension region, which were identical to those used in Example21. Nitrogen gas, from a cylinder, was metered into the throat of theextruder at a flow rate of ˜3 standard cubic feet per hour. The resinwas extruded at a rate of 229 lbs/hr and at a temperature of 220 deg. C.Following a brief induction period, there was no melt fracture on eithersurface of the tube.

EXAMPLE 23

This Example demonstrates the conditions under which melt fracture isencountered during blown film fabrication with PolypropyleneHomopolymers using conventional blown film dies.

A homopolymer of Polypropylene with the following characteristics wasobtained. The homopolymer had a nominal melt flow index of 1.5 and anIsotacticity index of 95.7. The homopolymer was formed into a tube bypassing the resin through a conventional 21/2 inch screw extruder havinga polyethylene screw as described in U.S. Pat. No. 4,329,313 with aMaddock mixing section, and thence into a conventional hard chromeplated steel die havaing a 1.375 inch land, 3 inch die collar diameterand a die pin diameter normally of 2.92 inches to give a 40 mil die gap.A Nitrogen purge at the throat of the extruder was used with a nitrogenflow rate of ˜3 standard cubic feet per hour. The sides of the die landwere parallel with the flow axis of the polymer melt. THe resin wasextruded through the die at a rate of 33.6 lbs/hr. The melt temperaturewas 218 deg. C. and the entire die was maintained at a temperature of207 deg. C. A 1.5 mil film was fabricated at these conditions. There wassevere surface melt fracture observed on both sides of the film.

EXAMPLE 24

This example demonstrates the improved results over Example 23 by theuse of Free Cutting brass die pin and collar under identical conditions.

The polypropylene homopolymer was identical to Example 23. Thehomopolymer was formed into a tube by passing the resin through theconventional 21/2 inch diameter extruder and mixer, along with thenitrogen purge on the throat, of Example 23 and into the die ofidentical design of Example 23 except that the die collar and pin wereconstructed from Free Cutting brass. The resin was extruded through thedie at a rate 34 lbs/hr. The melt temperature was 218 deg. C. and theentire die was maintained at a temperature of 204 deg. C. Following abrief induction period, there was no melt fracture observed on eithersurface of the tube.

EXAMPLE 25

This example demonstrates the conventional procedure (except for thenitrogen purge in the hopper) for extruding ethylene polymers into thinwall tubes.

An ethylene-butene copolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is available from UnionCarbide Corporation under Trademark designation Bakelite GRSN-7081 whichcontained 400 ppm of Kemamine AS 990. The copolymer had a nominaldensity of 0.918 gm/cc, a nominal melt index of 1.0 decigrams/minute anda nominal melt flow ratio of 26. The copolymer was extruded horizontallyto form a 1 inch tube with a wall thickness of 25 mils by passing theresin from a hopper which was purged with nitrogen at 3 standard cubicfeet per hour, through a conventional 21/2 inch diameter screw extruderhaving a polyethylene screw and thence into a conventional spider diehaving a 0.5 inch land, 1.25 inch collar diameter and a mandrel diameternormally of 1.225 inch diameter to give a 25 mil die gap. The sides ofthe die land region were parallel with the flow axis of the polymermelt. The resin was extruded through the die at a rate of 32.4 lbs/hr ata melt temperature at 216 deg. C. There was severe melt fractureobserved on both surfaces of the tube.

EXAMPLE 26

This example demonstrates the conventional procedure (except for thenitrogen purge in the hopper) for extruding ethylene polymers into thinwall tubes.

An ethylene-butene copolymer was prepared in accordance with theprocedure of U.S. Pat. No. 4,302,566 and which is available from UnionCarbide Corporation under Trademark designation Bakelite GRSN-7075 whichwas dry blended with 500 ppm of Kemamine AS 990. The copolymer had anominal density of 0.918 gm/cc. a nominal melt index of 0.5decigrams/minute and a nominal melt flow ratio of 26. The copolymer wasextruded horizontally to form a 1 inch tube with a wall thickness of 25mils by passing the resin from a hopper which was purged with nitrogenat 3 standard cubic feet per hour, through a conventional 21/2 inchdiameter screw extruder having a polyethylene screw and thence into aconventional spider die having a 0.5 inch land and a 1.25 inch collardiameter and a mandrel diameter normally of 1.225 inch to give a 25 mildie gap. The sides of the die land region was parallel with the flowaxis of the polymer melt. The resin was extruded through the die at arate of 42.6 lbs/hr at a melt temperature of 217 deg. C. There wassevere melt fracture observed in both surfaces of the tube.

EXAMPLE 27

This example demonstrates the improved results over Example 25 by theuse of Free Cutting Brass die collar and mandrel.

The ethylene-butene copolymer was identical to Example 25 and containedthe 400 ppm of Kemamine AS 990. The copolymer was extruded horizontallyto form a 1 inch tube with a wall thickness of 25 mils using theequipment and procedure identical to that in Example 25 except that thedie collar and mandrel was constructed from Free Cutting Brass. The diedesign was identical to that in Example 25. The resin was extrudedthrough the die at a rate of 32.4 lbs/hr at a melt temperature of 216deg. C. Following a brief induction period, there was no melt fractureon either surface of the tube.

Further, no melt fracture was observed on either surface of the tubewhen the copolymer was extruded at a rate of 70 lbs/hr.

EXAMPLE 28

This example demonstrates the improved results over Example 26 by theuse of Free Cutting Brass die collar and mandrel.

The ethylene-butene copolymer was identical to Example 26 and containedthe 500 ppm of Kemamine AS 990. The copolymer was extruded horizontallyto form a 1 inch tube with a wall thickness of 25 mils using theequipment and procedure identical to that in Example 26 except that thedie collar and mandrel was constructed from Free Cutting Brass. The diedesign was identical to that in Example 26. The resin was extrudedthrough the die at a rate of 42.6 lbs/hr at a melt temperature of 217deg. C. Following a brief induction period, there was no melt fractureon either surface of the tube.

The following example illustrates the improved properties of films fromnarrow gap Cu/Zn configurations over those from both the conventionalnarrow gap dies and the currently practiced wide gap technology (U.S.Pat. Nos. 4,243,619 and 4,282,177).

EXAMPLE 29

This example demonstrates the properties of 1.5 mil (38 micron) filmsproduced from GRSN-7047, formulated as in Example 1 and extruded atcomparabale conditions using a 3 inch spiral die fitted with: aconventional 40 mil gap of chrome plated steel as described in Example1; a 100 mil chrome plated gap as described in U.S. Pat. No. 4,282,177;and a 40 mil Free Cutting Brass configuration as described in Example 5.

    ______________________________________                                                     Die Gap, mil                                                                  40      100      40                                                           Design                                                                        Conven-          Free Cutting                                                 tional  Tapered  Brass                                           ______________________________________                                        Extrusion Rate, lbs/hr                                                                        66        66       66                                         Melt Temperature, deg. C.                                                                    221       220      220                                         Frostline Height, in.                                                                         21        21       21                                         Melt Fracture  Severe    None     None                                        Elmendorf Tear,                                                                           MD     29.9       86    165                                       gm/mil      TD     506       375    349                                       Dart Drop, gm/mil   48        82    117                                       Tensile Strength,                                                                         MD     7430      4810   3410                                      psi         TD     3310      3270   3090                                      Elongation, %                                                                             MD     420       562    514                                                   TD     752       652    642                                       Tensile Impact,                                                                           MD     1311      1647   1309                                      psi         TD     524       981    772                                       ______________________________________                                    

EXAMPLE 30

This example demonstrates the properties of 0.5 mil (12.5 micron) filmsproduced from DEX-7653, formulated as in Example 4 and extruded atcomparable conditions using a 3 inch spiral die fitted with: a 100 milchrome plated gap as described in U.S. Pat. No. 4,282,177; and a 40 milFree Cutting Brass configuration as described in Example 8.

    ______________________________________                                                      Die Gap, mil                                                                  100    40                                                                     Design                                                                        Tapered                                                                              Free Cutting Brass                                       ______________________________________                                        Extrusion Rate, lbs/hr                                                                         66       66                                                  Melt Temperature, deg. C.                                                                     229      229                                                  Frostline Height, in.                                                                          21       21                                                  Melt Fracture   None     None                                                 Elmendorf Tear,                                                                           MD      195      525                                              gm/mil      TD      820      764                                              Dart Drop, gm/mil   112      182                                              Tensile Strength,                                                                         MD      6410     7890                                             psi         TD      2090     3080                                             Elongation, %                                                                             MD      314      278                                                          TD      592      546                                              Tensile Impact,                                                                           MD      1539     1546                                             psi         TD      1382     1431                                             ______________________________________                                    

As will be seen from Examples 29 and 30, film products fabricated fromnarrow gap dies, fitted with Cu/Zn alloy surfaces for the die landregion, have been found to exhibit significantly improved criticalproperties, such as tear resistance and dart impact, and better balanceof properties in machine (MD) and transverse (TD) directions.

What is claimed is:
 1. A process for substantially eliminating surfacemelt fracture during extrusion of a thermoplastic polymer underconditions of flow rate and melt temperature which would otherwiseproduce such surface melt fracture which comprises extruding saidpolymer through a die having a die land region defining opposingsurfaces, and wherein at least one of said opposing surface isfabricated from an alloy containing 5 to 95 parts by weight zinc and 95to 5 parts by weight of copper whereby surface melt fracture issubstantially eliminated on the surface of the polymer adjacent to saidzinc/copper containing alloy surface.
 2. A process according to claim 1wherein said thermoplastic polymer is a molten narrow molecular weightdistribution, linear, ethylene polymer or copolymer.
 3. A processaccording to claim 2 wherein a stabilizing additive is added to saidethylene copolymer.
 4. A process according to claim 3 wherein saidstabilizing additive is a fatty diethoxylated tertiary amine.
 5. Aprocess according to claim 4 wherein said fatty diethoxylated tertiaryamine is added to said ethylene copolymer in an amount of about 50 to1500 parts per million.
 6. A process according to claim 2 wherein saidalloy includes tin, or aluminum or lead or mixtures thereof.
 7. Aprocess according to claim 2 wherein said alloy contains about 30 to 40parts by weight zinc and about 70 to 60 parts by weight copper.
 8. Aprocess according to claim 2 wherein said alloy surface in said die landregion is provided by inserts secured to the pin and collar of said die.9. A process according to claim 8 wherein said inserts extend the lengthof said die land region.
 10. A process according to claim 8 wherein saidinserts extend for a portion of the length of said die land region. 11.A process according to claim 2 wherein said alloy surface is provided byfabricating the die pin and die collar of said die from said alloy. 12.A process according to claim 2 wherein the distance between said dielips is up to about 0.050 inch.
 13. A process according to claim 2wherein said copolymer is a copolymer of greater than or equal to 80 molpercent of ethylene and less than or equal to 20 mol percent of at leastone C₃ to C₈ alpha olefin.
 14. A process according to claim 13 in whichsaid copolymer has a melt index of greater than or equal to 0.1 to lessthan or equal to 5.0.
 15. A process for substantially eliminatingsurface melt fracture during extrusion of a molten narrow molecularweight distribution, linear, ethylene copolymer, under conditions offlow rate and melt temperature which would otherwise produce suchsurface melt fracture which comprises extruding said polymer through adie having a die land region defining opposing surfaces and wherein atleast one of said opposing surface is fabricated from an alloycontaining about 30 to 40 parts by weight zinc and about 70 to 60 partsby weight copper said ethylene copolymer containing from about 50 to1500 ppm of a fatty diethoxylated tertiary amine, whereby melt fractureis substantially eliminated on the surface of the polymer adjacent tosaid zinc/copper containing alloy surface.
 16. A process according toclaim 15 wherein said alloy surface in said die land region is providedby inserts secured to the pin and collar of said die.
 17. A processaccording to claim 11 wherein said inserts extend the length of said dieland region.
 18. A process according to claim 16 wherein said insertsextend for a portion of the length of said die land region.
 19. Aprocess according to claim 15 wherein said alloy surface is provided byfabricating the die pin and die collar of said die from said alloy. 20.A process according to claim 1 wherein said thermoplastic polymer is apropylene polymer.
 21. A process according to claim 20 wherein saidpropylene is a copolymer of greater than or equal to 85 mol percentpropylene and less than or equal to 15 percent of ethylene and/or one ormore of C₃ to C₈ alpha olefins.
 22. A process according to claim 20wherein said propylene polymer is a block or impact copolymer with atotal weight percent of from about 70 to about 95% of propylene and aminor weight percent of from about 5 to about 30% of ethylene and/or oneor more C₃ to C₈ alpha olefins.
 23. A process according to claim 1wherein said resin is subjected to an inert gas to remove moisturecontent prior to extrusion.
 24. A process according to claim 23 whereinsaid inert gas is nitrogen.
 25. A die for extruding thermoplasticpolymers prone to melt fracture which comprises a die block having inletmeans for introducing an extruder melt into said die block and outletmeans for discharging molten polymer from said die block, conduit meansfor passing molten material from said inlet means to said outlet means,said outlet means including a land entry section and a die land, saiddie land having surfaces in contact with said molten polymer, at leastone of said surfaces being fabricated from an alloy containing 5 to 95parts by weight zinc and 95 to 5 parts by weight of copper whereby meltfracture is substantially eliminated on the surface of the polymeradjacent to said zinc/copper containing alloy surface.
 26. A dieaccording to claim 25 wherein said alloy includes tin, or aluminum orlead or mixtures thereof.
 27. A die according to claim 25 wherein saidalloy contains about 30 to 40 parts by weight zinc and about 70 to 60parts by weight copper.
 28. A die according to claim 25 wherein saidalloy surface in said die land region is provided by inserts secured tothe pin and collar of said die.
 29. A die according to claim 28 whereinsaid inserts extend the length of said die land region.
 30. A dieaccording to claim 28 wherein said inserts extend for a portion of thelength of said die land region.
 31. A die according to claim 25 whereinsaid alloy surface is provided by fabricating the die pin and die collarof said die from said alloy.
 32. A die according to claim 25 wherein thedistance between said die lips is between about 0.005 inch to about0.040 inch.